1. Technical Field
This disclosure relates to the generation and storage of hydrogen fuel.
2. Related Art
Hydrogen is a renewable fuel that produces zero emissions when used in a fuel cell. In 2005, the Department of Energy (DoE) developed a new hydrogen cost goal and methodology, namely to achieve $2.00-3.00/gasoline U.S. gallon equivalent (gge, delivered, untaxed, by 2015), independent of the pathway used to produce and deliver hydrogen. The principal method to produce hydrogen is by stream reformation. Nearly 95% of the hydrogen currently being produced is made by steam reformation, where natural gas is reacted on metallic catalyst at high temperature and pressure. While this process has the lowest cost, four pounds of the greenhouse gasses carbon monoxide (CO) and carbon dioxide (CO2) are produced for every one pound of hydrogen. Without further costly purification to remove CO and CO2, the hydrogen fuel cell cannot operate efficiently.
Devices that are configured to electrochemically convert water into hydrogen and oxygen when energy is applied are generally known as water electrolyzers. Presently, about five percent of hydrogen production comes from water electrolysis. This reaction comprises the direct splitting of water molecules to produce hydrogen and oxygen. Importantly, greenhouse gasses are not produced in these reactions. In this process, electrodes comprising catalyst particles are submersed in water, and energy is applied to the electrodes. Using this energy, the electrodes split water molecules into hydrogen and oxygen. Hydrogen is produced at the cathode electrode, which accepts electrons, and oxygen is produced at the anode electrode, which liberates electrons. The amount of hydrogen and oxygen produced by an electrode is dictated by the current supplied to the electrodes. The efficiency depends upon the voltage between the two electrodes, and is proportional to the reciprocal of that voltage. In other words, the efficiency of the system increases as the voltage decreases. A more catalytic system will have a lower voltage for any one current, and therefore be more efficient in producing hydrogen and oxygen. If the catalyst is highly efficient, there will be minimal energy input to achieve a maximum hydrogen output. While this process is currently too expensive to compete with steam reformation due low efficiency and the use of expensive catalysts in the electrodes, emerging technologies show promise in balancing the economies.
For an electrolyzer to operate with high efficiency, the amount of product produced during reaction should be maximized relative to the amount of energy input. In many conventional devices, significant efficiency loss stems from low catalyst utilization in the electrodes, cell resistance, inefficient movement of electrolyte, and inefficient collection of reaction products from the electrolyte. In many cases, low efficiency is compensated for by operating the cell at a low rate (current). While this strategy increases efficiency, it also lowers the amount of products that can be produced at a given time.
The high-purity hydrogen produced in any of the above methods can be compressed and stored in either compressed gas cylinders, liquefied to liquid hydrogen, or adsorbed in solid state storage systems, such as metal hydride storage systems. The fuel is then accessible later for power generation. Solid state storage of hydrogen is widely viewed as a practical strategy for compact hydrogen storage. The principle is directly used in rechargeable batteries such as nickel-metal hydride (NiMH) batteries, in which hydrogen is reversibly absorbed into the anode electrode during battery cycling. Metal hydrides are also used as a source of hydrogen for supply of the said hydrogen to hydrogen fuel cells. These metal hydrides are inherently safe, and have good specific energy (˜280 Wh/l) and energy density (˜80 Wh/kg).
Hydrogen has been shown to be a tremendous fuel source for, amongst other energy (power) generators, fuel cells. A fuel cell is a device that converts chemical energy directly into electrical energy, via consumption of a fuel, such as hydrogen, an alcohol, or other hydrocarbons. The fuel cell comprises a negative terminal (anode), where the hydrogen fuel is consumed, and a positive terminal (cathode), where oxygen fuel is consumed. This energy generating device is highly advantageous in that fuel can be resupplied; the device will operate as long as anode and cathode are supplied with fuel. The anode fuel is oxidized on a catalyst surface to produce electrons and ions. Ions flow through an ion exchange membrane, and the electrons flow through an external circuit, generating electricity. Electrons and ions then recombine at the cathode catalyst surface with the cathode fuel. At the core of the fuel cell is the membrane-electrode assembly (MEA). The MEA comprises a membrane capable of exchanging ions such as H+ or OH−, a catalyst layer applied to each side of the membrane, and an electrically conductive backing on each catalyst layer. Reliable adhesion and interaction between these layers are some important factors for a fuel cell to operate at the highest power. To promote excellent catalyst utilization as well as electronic and ionic flow within the fuel cell, the composition and interfaces of the catalyst layer are some important factors to achieving low ohmic resistance and increased power output.
Electrodes for use in fuel cells and/or electrolyzers can comprise nano-metal particles, or a combination of nano- and micro-metal particles, that can be either be sintered as a monolithic structure, or applied as a layer (or embedded into) a supporting substrate structure, such as is described in Provisional Application Ser. No. 61/109,453, filed on Oct. 29, 2008, Provisional Application Ser. No. 61/046,790, filed on Apr. 21, 2008, U.S. application Ser. No. 11/868,152, filed on Oct. 5, 2007, U.S. application Ser. No. 12/114,719 filed on May 2, 2008, which has priority to Provisional Application No. 60/915,619, filed on May 2, 2007, U.S. application Ser. No. 12/053,484, filed on Mar. 21, 2008, which has priority to Provisional Application Ser. No. 60/896,722, filed on Mar. 23, 2007, U.S. application Ser. No. 11/781,909, filed on Jul. 27, 2007, which is a continuation-in-part of and has priority to U.S. application Ser. No. 11/394,456, filed on Mar. 31, 2005, U.S. Ser. No. 11/482,290, filed on Jul. 7, 2006, and U.S. Ser. No. 11/525,469, filed on Sep. 22, 2006, the entire contents of all of which are expressly incorporated herein by reference.
The composition of the metal nanoparticles can be a pure metal, an oxide of a metal, or an alloy of two or more metals. Preferably, the metal composition is selected from groups IIA, IB, and IIIB-VIIIB of the periodic table, most preferably nickel, manganese, aluminum, cobalt, copper, tin, palladium, silver, gold, lanthanum, and alloys thereof. Other metals have been shown to prove useful as a catalyst in an electrochemical context. The nano-metal particles can be made from one of a number of manufacturing process, such as the ones described in U.S. Pat. No. 7,282,167 to Carpenter issued on Oct. 16, 2007, and U.S. Ser. No. 11/591,787, filed on Nov. 2, 2006, the entire contents of both of which is hereby expressly incorporated by reference. In the '167 patent and '787 application, processes for making nano-metal particles more uniformly are described.
Portable power generation has increased significantly in demand as portable consumer electronics become personal assistants for most forms of entertainment and information. Powering these portable consumer electronics for long term use presents challenges, most of which are being addressed as part of battery technology. Batteries can comprise single use configurations or rechargeable configurations. While battery technology has proven to be quite successful, alternative sources of energy are also being considered. Indeed, technology has emerged that permits electrochemical systems to be scaled down to address desires of greater portability that eliminate the need for a traditional battery.
Others have suggested powering portable consumer electronics with a fuel cell supplied with a continual source of chemical energy to power the fuel cell. The fuel cell works in place of a battery by providing an efficient supply of electric power to the consumer electronic device. Like a battery, however, a supply of chemical energy is necessary. Therefore, a solution is desired to effectively provide a portable source of chemical energy to more easily and readily generate electric power for portable consumer electronics.